linux-kernel - Re: [PATCH 2/3] asm-generic, x86: wrap atomic operations

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Date:   Tue, 28 Mar 2017 11:51:51 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 2/3] asm-generic, x86: wrap atomic operations


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Mar 28, 2017 at 09:52:32AM +0200, Ingo Molnar wrote:
> 
> > No, regular C code.
> > 
> > I don't see the point of generating all this code via CPP - it's certainly not 
> > making it more readable to me. I.e. this patch I commented on is a step backwards 
> > for readability.
> 
> Note that much of the atomic stuff we have today is all CPP already.

Yeah, but there it's implementational: we pick up arch primitives depending on 
whether they are defined, such as:

#ifndef atomic_read_acquire
# define atomic_read_acquire(v)		smp_load_acquire(&(v)->counter)
#endif

> x86 is the exception because its 'weird', but most other archs are
> almost pure CPP -- check Alpha for example, or asm-generic/atomic.h.

include/asm-generic/atomic.h looks pretty clean and readable overall.

> Also, look at linux/atomic.h, its a giant maze of CPP.

Nah, that's OK, much of is is essentially __weak inlines implemented via CPP - 
i.e. CPP is filling in a missing compiler feature.

But this patch I replied to appears to add instrumentation wrappery via CPP which 
looks like excessive and avoidable obfuscation to me.

If it's much more readable and much more compact than the C version then maybe, 
but I'd like to see the C version first and see ...

> The CPP help us generate functions, reduces endless copy/paste (which induces 
> random differences -- read bugs) and construct variants depending on the 
> architecture input.
> 
> Yes, the CPP is a pain, but writing all that out explicitly is more of a
> pain.

So I'm not convinced that it's true in this case.

Could we see the C version and compare? I could be wrong about it all.

Thanks,

	Ingo